1. Field
The present invention relates to capacitive transducers, especially techniques for processing input signals from a capacitive transducer structure into a digital output signal.
2. Description of the Related Art
Micro-Electro-Mechanical Systems or MEMS can be defined as micro-scale mechanical and electro-mechanical systems where at least some elements have a mechanical functionality. MEMS structures can be applied to quickly and accurately detect very small changes in physical properties.
The vast majority of electronic devices today use the MEMS sensors to detect some specific physical phenomenon. The conditioning and processing of the electrical signal from the sensor is an important stage, since the generated signals can be quite small, and are easily corrupted by ambient interferences. Many MEMS sensors apply capacitive transducers to transform events from the physical domain into electrical signals that are suitable for further processing. In most cases, the output from the capacitive transducers requires amplification and conditioning to provide the best possible signal for analog-to-digital conversion. A combination of conditioning circuitry is often called an analog front end. The analog front end may also include the circuitry for the analog-to-digital conversion.
Capacitive transducers in MEMS sensors often apply differential detection with two capacitors. For differential detection, in response to a detected activity, a first capacitor of a capacitor pair generates a first input signal, and a second capacitor of the capacitor pair generates a second input signal. The first input signal and the second input signal may be detected in parallel and processed in combination for added accuracy.
A conventional measurement principle applied with differential detection has been a self-balancing capacitor bridge where the capacitive transducer consists of a movable plate with a fixed electrode on each side. Together the three electrodes form two capacitors. Deflection of the plate is normalized with respect to the distance between the fixed electronics. The normalization of the self-balancing capacitor bridge provides a linear and stable transfer function, but higher signal-to-noise levels are required for many modern applications.
A number of further methods to improve the signal-to-noise levels in capacitance-to-voltage conversion have been introduced. For example, correlated double sampling and/or chopping techniques have been combined to active or passive common mode charge cancellation. These solutions provide improved signal-to-noise levels, but their output is directly proportional to the differential sensor capacitance. However, in the absence of normalization, the linearity in gap modulated capacitive electrodes in MEMS is reduced. In addition, the circuit tends to be sensitive to possible mismatch between MEMS capacitors in the capacitive transducer and ASIC capacitors in the analog front end circuit.